1. Field of the Invention
The present invention is broadly concerned with compositions and methods for sorbing and/or destroying dangerous substances such as chemical and biological warfare agents and environmental pollutants in air, water, and soil. The methods of the invention are carried out by simply contacting the target substance with metal oxide nanoparticles coated with a coating material selected from the group consisting of oils, surfactants, waxes, silyls, polymers (both synthetic and natural), epoxy resins, and mixtures thereof.
2. Description of the Prior Art
The extremely high surface reactivity of a variety of nanocrystalline inorganic oxides has been well documented (see e.g., U.S. Pat. Nos. 6,093,236, 6,057,488, 5,990,373, each incorporated by reference herein). These patents demonstrate their use as destructive absorbents for various toxic materials, including acid gases, air pollutants, and chemical and biological warfare agents. While there can be no doubt about the emerging popularity of nanoparticles as superadsorbents, one significant drawback for some is their sensitivity to air exposure that results in appreciable reactivity loss. For example, magnesium oxide nanoparticles typically undergo the following changes upon exposure to humid air (50-55% RH, room temperature, 24 hours):                weight gain of 45-60%;        large decrease in surface area (from 500-600 m2/g to 40-50 m2/g;        change in pore structure                    pore diameter (from 35-91 Å to 107-319 Å);            pore volume (0.5-0.9 cc/g to 0.2-0.3 cc/g);            partial conversion to hydroxide as demonstrated by IR and XRD analyses (see FIGS. 1 and 2);            some carbonate formation as illustrated by IR analysis (see FIG. 1);            reduced reactivity towards paraoxon (see FIG. 3);                            nanocrystalline magnesium oxide (0.2 g) adsorbs 9 μL of paraoxon in about 3 minutes;                humidified nanocrystalline magnesium oxide particles adsorb only 40-50% of this amount even after 20 hours.                                                
Thus, there is a need for improved nanocrystalline metal oxide adsorbents which do not lose their adsorbent properties upon exposure to air. Furthermore, these adsorbents should have a coating material which tends to exclude air (water, carbon dioxide, etc.) while allowing the target compound to contact and penetrate the coating so that the target compound will contact the reactive nanoparticle metal oxide.